Copper Tin

Overview

Copper tin alloys or tin bronzes are known for their corrosion resistance. Tin bronzes are stronger and more ductile than red and semi red brasses. They have high wear resistance and low friction coefficient against steel. Tin bronzes, with up 15.8% tin, retain the structure of alpha copper. The tin is a solid solution strengthener in copper, even though tin has a low solubility in copper at room temperature. The room temperature phase transformations are slow and usually do not occur, therefore these alloys are single phase alloys. The tin bronzes are used in bearings, gears, piston rings, valves and fittings. The cast tin bronzes are designated by UNS C90200 through C91700. Lead is added to tin bronzes in order to improve machinability and pressure tightness. Lead decreases the tensile strength and ductility of the tin bronzes, but the composition can be adjusted to balance machinability and strength requirements. High leaded tin bronzes are primarily used for sleeve bearings. These alloys have a slow fail mechanism that temporarily prevents galling and seizing. The slow fail mechanism works by lead seeping out of the alloy and smearing over the surface of the journal. The cast leaded tin bronzes are designated as UNS C92200 through C94500.

The microstructure of the cast tin bronzes consists of cored dendrites, they have a composition gradient of increasing tin as they grow. The last liquid to solidify is enriched with tin upon cooling, and forms alpha and delta phases. The alpha and delta phases fill in the areas between the dendrite arms. the microstructure of the leaded tin bronzes are similar to the nonleaded materials with the addition of lead particles in the inter-dendritic boundaries. The lead is practically insoluble in solid copper and it solidifies last as almost pure lead in the grain boundaries.

NOTE: The file size of the Larger and Largest View of the Micrographs are substantially larger than the thumbnail shown. The Larger View images range in size from 11K to 120K depending on the image. The Largest View images range in size from 125K to almost 500K.